This application claims priority Japanese Patent Application No. 2001-342633, filed Nov. 8, 2001 in Japan, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates generally to method of reducing low-frequency noise for semiconductor devices and circuits. The circuits contains cryogenic transistors that operate at a cryogenic temperature are housed in a cryostat, the cooling temperature of which is adjustable. Moreover the present invention relates specifically to a method of reducing low-frequency noise in transistors used in cooled readout circuits that are used for high-sensitivity, high-resistance cryogenic photodetectors.
2. Description of the Related Art
The related art is as follows. Heretofore, cryogenic field effect transistors (FETs) have been used as electronics of trans-impedance devices for feeble light sensors. FETs (GaAs junctin FETs, for example) used in the circuits at cryogenic temperature tend to cause low-frequency noise that adversely affects the performance of the readout circuit for the photodetectors. Now, random telegraph signals (RTS) when a GaAs junction FET (JFET) is operated at cryogenic temperatures are explained in the following.
FIGS. 20A and 20B are drawings to explain problems in the related art. FIG. 20A illustrates an output waveform of GaAs J-FET and FIG. 20B illustrates the temperature dependence of the noise spectra. In FIG. 20A, the abscissa represents time (sec) and the ordinate represents the output xc3x97102 (V). FIG. 20A shows a noise output in a certain range of time under a steady state. In FIG. 20B, the abscissa represents the frequency (Hz) and the ordinate represents the gate input referred noise spectrum (dBV/Hz1/2). The V/Hz1/2 mentioned above is equal to V/Hz, and this applies to the following description throughout. Some sharp signals of the noise found in FIG. 20A are RTSs. Note that sharp three peaks appearing in the vicinity of 20-70 Hz in FIG. 20B are power noise that are different from the RTS noise with which the present invention is concerned.
The RTS is a discrete conductance fluctuation found in the operation of a solid-state device. The RTS is characterized in that its spectra closely resemble those of Generation-Recombination noise (G-R noise), however the amount of its conductance fluctuations is constant, which is different from the G-R noise. Owing to the constant fluctuation of its conductance, the RTS is generally considered that it is caused from switching (or transition) of a single electron to a trap level, but its detailed physical process is not yet to be elucidated. The switching of a single electron to a trap level affects to total currents of the transistor so as to be modulated with the trap level whether the trap is occupied with an electron or not.
The RTS was reported occurring in metal-oxide-semiconductor FETs (MOSFET), J-FETS, Quantum Well Infrared Photoconductors (QWIPs), bipolar pn-junction transistors, etc. of various structures and materials, or under various operating conditions (refer to K. Kandiah IEEE Trans. Electron Devices, vol. 41, p. 2006, 1994, for example.)
The RTS might cause serious deterioration of performance when FETs are used in a readout circuit in a high-sensitivity sensor where the system performance is limited by low-frequency noise. The static characteristics of GaAs J-FETs and the dependence of noise on their operating conditions were investigated in a temperature range of 2Kxcx9c77K (K: Kelvin). The results are as follows:
The typical gate size of the FETs used was W/L=5 xcexcm /50 xcexcm (W: width, L: length). It can be concluded from the noise spectra and output waveforms measured at various operating voltages that the low-frequency noise generated under the cryogenic condition of GaAs J-FETs are noise predominantly caused from the RTS noise. In FIG. 20B, Lorentzian spectra are seen, and it can be observed that the cutoff frequency increase as the operating temperature rises over 40K, while independent of the operating temperature between 4.2 and 20K.
FIG. 20A shows an output waveform at 4.2K of a GaAs J-FET when operated in a saturation region of VD (drain-source voltage)=0.7 5V and VG (gate voltage)=0.32V. FIG. 20B shows the temperature dependence of noise spectra in the same FET when operated under the same voltage conditions. The drain current in this case is 2.4 xcexcA.
On the basis of the wave height in FIG. 20A, the amount of discrete current variation can be estimated at 1.5 nA, which amounts to about {fraction (1/1000)} of the entire current. As can be seen in FIG. 20B, the incidence rate of RTSs varies depending on temperature, while little changes can be observed in the variance of carrier residence time in the trap. That is, at temperatures above 45K or more than 45K, noise spectra show temperature dependence, and the Lorentzian noise spectrum shifts to the higher-frequency side with increases in temperature. In other words, the process of electron transition to the trap level causing RTSs is dominated by thermal fluctuations in the vicinity of 45K or higher.
The aforementioned prior art has the following problem. As described earlier, it was concluded that low-frequency noise in a GaAs J-FET at cryogenic temperatures are dominated by noise arising from RTSs. The RTS may cause serious deterioration of performance when FETs are used in the readout circuit of a high-sensitivity sensor where low-frequency noise may limit system performance. An object of the present invention is to solve the aforementioned problems with the prior art and to reduce low-frequency noise in cryogenic semiconductor devices.
To achieve the aforementioned objectives, the present invention has a following method. In a method for reducing low-frequency noise in a cooled readout circuit where a cryogenic transistor is housed in a cryostat the cooling temperature of which is adjustable, as a first step, the transistor is put into the operating state at a first temperature by turning on the power. If RTSs are seen in the output waveforms, the temperature of the transistor is raised to higher than the first temperature, while flowing current in the semiconductor device. As the second step, the temperature of the transistor is cooled down from the second temperature to a cryogenic temperature again. Upon completion of the second step, the transistor is operated at the cryogenic temperature as the third step. In this way, the present invention reduces the low-frequency noise.
Another aspect of the present invention is in the following. In a method for reducing low-frequency noise in a semiconductor device operating in a cryogenic temperature, as a first step, a semiconductor device is turned on at a cryogenic temperature, and the temperature of the semiconductor device is temporarily raised to a second temperature that is higher than the first temperature, while flowing current in the semiconductor device. Following the first step, as a second step, the semiconductor device is cooled from the second temperature to a cryogenic temperature again. The transistor is operated at the cryogenic temperature.
The objects, advantages and features of the present invention will be more clearly understood by reference to the following detailed disclosure and the accompanying drawings.